Here is an interesting interview of some Osram OS employees from 12-2016 (in German).
They talk about Osram Stage LEDs. The white variants of these LEDs have new kind of ceramic-phosphor on top. It does not contain silicone and instead is sintered, compressed and baked at 1000°C. It is supposed to tolerate higher temperatures better and does not show signs of degradation.
They also consider 2mm2 die size at high-currents to be an innovation.
The UX:3 technology is a different way of getting the current to the die. Instead of putting a wire mesh on top (which reduces light output and adds electrical resistance), they input current from the bottom side and spread it inside the semiconductor. This also results in a more even current distribution.
EDIT: I found it, the paper regarding the development of the UX:3 technology: Development of high-efficiency and high-power vertical light emitting diodes (2014).
Generally this technology seems to have been a very big step for Osram. It caused big jumps in performance and the current Black Flat and Q8WP are definitely already highly optimized LEDs for high luminance and reliability.
The lower resistance of the current spreader caused a reduction in Vf of the LEDs compared to older models.
Interesting statement: "The maximum power of the devices is only limited by the resistivity of the current distribution layer and the current density of the n-contacts, which can both be engineered according to the application requirements."
Another one: "Owing to the low-Rth(<1.4 K/W for a 1mm2 chip) design, formed by a silicon carrier with solder die attached, the maximum current can be maintained up to 150 °C junction temperature."
That thermal resistance is a lot lower than the value in the datasheet of the Black Flat.
The LE UW Q8WP seems to be the 2mm2 SHP (super high power) chip they talk about there, the largest variant of the technology at that point. It looks exactly like their diagrams (except for only being 1.8mm2).